Thin films (and thick films in some cases) are useful for forming many different electronic and optical devices. Capacitors (and other devices such as phase shifters) are formed by depositing dielectric material and conductive material layers, the conductive layer(s) forming the electrodes of the capacitor. In optical devices, films of material having relatively high indexes of refraction are deposited adjacent to films of materials having relatively low indexes of refraction to form wave-guides, filters, lenses and other devices. Many materials are useful for forming both the dielectric material as well as the materials having high indexes of refraction.
Also, in the fields of electronics, RF and photonics, the use of electrically active materials is becoming more and more popular. Electrically active materials are those materials that have a particular parameter that can be changed by applying an electric field through the material. This parameter may be optical such as the index of refraction of the material (electrooptic, E-O, materials), or may be electrical such as the dielectric constant (ferro (or para) electric). As these two parameters are associated, some materials are both ferroelectric and electrooptic.
The class of dielectric materials that possess the property tat their permittivity (dielectric constant—DK) changes under the application of a DC or slowly varying electric field are commonly referred to as “ferroelectrics” (f-e) if the material is designed to operate below the materials Curie temperature or point, or “paraelectrics” (p-e) if the material is designed to operate above its Curie point. For simplicity, they will be called ferroelectrics (f-e) in this disclosure, and it will be understood to apply to either f-e or p-e materials. Of particular interest for microwave device applications is the paraelectric range of a material, i.e., where the material is above its Curie temperature. In the paraelectric region the variation in capacitance of the material is substantially linear with respect to applied bias voltage. Barium strontium titanate having the formula BaxSr1-x-TiO3 (BST) is a highly studied niateiial with great potential in these applications. BST is used herein to refer to material having the formula BaxSr1-xTiO3 where x equals 0, x equals 1, or x is greater than 0 and less than 1. For SrfiO3 (x equals 0) the Curie temperature is very low, and this material is in the paraelectric range at cryogenic temperatures. For BaTiO3 (x equals 1) the Curie temperature is high. Generally an intermediate Curie temperature is desired; thus for most applications, the molar ratio of Ba:Sr is between 10:90 and 90:10, particularly between 30:70 and 70:30, more particularly between 60:40 and 40:60. The choice of Ba:Sr, and thereby the selection of Curie temperature, is thus selected according to the anticipated operating temperature of the device. For devices intended to be operated in a range encompassing room temperature, a Ba:Sr molar ratio of between 60:40 and 40:60 is preferred. Mixed oxides, such as BST where x is greater than 0 and less than 1 are harder to deposit than single cation oxides, such as when x is 0 or 1. This difficulty is primarily compositional control. The combustion chemical vapor deposition process, as described for example, in U.S. Pat. No. 5,652,021, the teachings of which are incorporated herein by reference, allows the use of a single precursor solution containing precursors for all cations. A single solution significantly aids in compositional control, although it is important to maintain other depositional parameters, such as temperature, pressure, gas flow rates, etc. in order to maintain compositional control even in a CCVD process. Preferably, in a mixed BST, the ratio of B/S does not vary spatially more than about 5%, preferably no more than about 1% during the deposition of a layer.
Electrical, radio frequency (RF), or microwave applications of these electrically active materials include such general classifications as varactor diode replacement, capacitors, tunable capacitors, tunable filters, phase shifters, multiplexers (to include duplexers), voltage controlled oscillators, tunable matching networks for power amplifiers (PA's), low noise amplifiers (LNA's), thermoelectric effects including power systems, and general impedance matching networks.
The tunable characteristic of f-e materials can be exploited in the design of components, subsystems and/or systems in mobile communication systems to achieve:
1) new capability and improved electrical (RF or microwave) performance from 300 MHz to ˜30 GHz
2) smaller size,
3) lower power consumption,
4) less weight,
or any combination of these four items as determined by specific system design requirements.
There are numerous ceramic materials that can be used as f-e thin or thick films. Thin films tend to be used in smaller devices than thick films; thin films are generally deposited to a thickness up to 10 microns, while thick films are typically above 10 microns.
Wireless handsets are characterized by their need for low voltage operation, typically <40 VDC, and ideally <3.0 VDC. It is expected that this voltage will decrease further in future designs. Thus, any f-e tunable device must be able to be designed in such a way as to create appropriate electric fields from a small DC tuning voltage. One way to achieve a suitable geometry is to design variable capacitors consisting of thin films of f-e materials. The small DC tuning voltage also results in reduced power consumption (and heat dissipated) from RF and E-O devices.
Tunable capacitors allow for the f-e material to be localized to a small part and allows for the use of the small geometries needed to create electric fields of sufficient magnitude necessary for tuning from small voltages. For the design of tunable filters and multiplexers in the frequency range of >800 MHz (the cellular band), small valued capacitors are required so that the rf signal is not reduced if the variable capacitor is used in such a way as to shunt a resonant structure for tuning purposes.
As previously described, related to the variation in capacitance of these materials with applied bias voltage is the electrooptic phenomena of variation of refractive index with applied bias voltage. Photonic applications of these materials are in phase modulators and active waveguides that have functions such as switch, split, attenuate, compensate or combine.
Barium Strontium Titanate (BST) is a useful material for the above applications. BST is also used herein to refer to doped material wherein an additional element(s), such as lead, replaces some of (usually less than 15%, and more commonly less than 10%, but even up to 50%) of the Barium or Strontium in the crystal lattice. Alternatively, elements such as tungsten, aluminum, magnesium, calcium and others can be used to modify the properties of the BST by replacing some of the Titanium in the lattice. Such dopants may improve the Q factor of the BST. As Ba and Sr have +2 valences, typical valences also have +2 valences. However, combinations of +3,+3, and/or +1/+5 valence doping ion combinations may be used. Cations of valence other then +2 may be used by themselves in BST with vacancies in the lattice structure. Cesium and Bismuth are such dopants. BST is a recognized ferroelectric and BaTiO3 is a known E-O material for the applications described above. BST can be doped by most metallic elements.
To eliminate grain boundaries that create loss in both optical and electrical devices, epitaxial f-e materials are highly preferred to polycrystalline f-e materials. Heretofore, the most common substrate material for epitaxial BST deposition has been magnesium oxide and lanthanum aluminum oxide, materials which have crystal lattice structures which match that of BST. A significant drawback of both magnesium oxide and lanthanum aluminum oxide is that they are currently available only in very small wafer sizes. Furthermore, these small wafers are very expensive to produce. They also tend to exhibit poor crystallinity and poor surface roughness.
There are implicitly huge benefits to be realized from larger wafers. The semiconductor industry is currently moving toward 12 inch square (30 cm. square) wafer sizes so as to realize lower production costs. Similar cost savings can be implicit with ceramic materials available in a larger size for microelectronic device fabrication.
Currently, sapphire is available in 100 mm wafers with some suppliers planning to introduce 150 mm wafers soon. This is significantly larger than available wafer sizes for magnesium oxide and lanthanum aluminum oxide.
Sapphire, single crystal alumina, has recognized benefits over both magnesium oxide and lanthanum aluminum oxide in that it can be produced at lower cost, increased wafer size, excellent crystallinity and minimum surface roughness. However, BST is not an obvious crystal lattice match, and, indeed, attempts to date to deposit epitaxial BST on sapphire have not met with success.
For some applications, less than epitaxial quality BST and other materials may be used. Co-pending PCT Application PCT/US01/22131, entitled “REDUCED GRAIN BOUNDARY CRYSTALLINE THIN FILMS”, filed Jul. 13, 2001, describes methods of forming reduced grain boundary (RGB) thin films that are useful in forming some of the devices of the present application. For applications that require highly epitaxial films, reference should also be made to Co-pending PCT Application PCT/US01/24587, entitled “ELECTRONIC AND OPTICAL MATERIALS”, filed Aug. 2, 2001, which discloses methods for producing epitaxial thin films useful in forming the devices of the present invention. It should also be noted that some of the thin films in the devices of the present invention may still operate adequately in polycrystalline or even amorphous form.